1. Field of the Invention
The present invention relates to fuel system and method for providing fuel to a combustion chamber of an engine for combustion therein, in particular in which the fuel is preheated at moderate pressures to maximize vaporization in the combustion chamber, and in which substantially leaner fuel air mixtures are provided than hitherto, wherein substantial advantages in fuel consumption are achieved. The present invention relates to such a system also comprising means for providing combustion stability for improving engine life.
2. Prior Art
In an internal combustion engine, a fuel/air mixture necessary for the combustion process in the combustion chamber of each cylinder is provided typically by a fuel injection system or a carburetor upstream of or within the inlet manifold, the combustible mixture comprising droplets of fuel of differing sizes entrained in a stream of air. As is well known, at relatively lower temperatures, fuel droplets tend to be of larger diameter and less homogeneously distributed in the air stream than at relatively higher temperatures.
The fuel entry point (typically by way of the carburetor or fuel injector) is generally distanced from the intake port of each combustion chamber by a length of ducting. Typically, the fuel droplets, being distanced away from the hot walls, are kept relatively cool inhibiting full fuel vaporization, and further, the effect of the air stream enhances mutual absorption of droplets into larger droplets. The result is that the fuel/air mixture reaching the combustion chamber comprises a substantially fuel-rich centrally flowing portion comprising a high proportion of fuel droplets that cannot combust rapidly enough when ignited because of their relatively large size, and thus relatively large volume: surface area ratio, and poor availability of oxygen due to non-homogeneous mixing of the air and fuel. The higher the engine rpm, the greater the tendency for the fuel to migrate to the centre of the air stream.
Thus, a proportion of the fuel, typically between 10% and 30% or even higher, is not properly utilized by the engine for generating power, and remains unburnt or is burnt late in the power cycle, being at least partially transformed into pollutants that, to avoid being discharged into the atmosphere, require expensive catalytic converters in the exhaust system for their neutralization. Further, the incomplete combustion of the fuel also results in the formation of carbon deposits, reducing the service life of the ignition units, pistons, valves and the engine in general.
One attempt at increasing the combustion efficiency has focused on pre-heating the fuel prior to injection or to carburetion. Many patents on the subject discuss atomization and/or partial vaporization of fuel. Vaporization systems relate to systems in which the fuel is vaporized, i.e., transformed into the gaseous state of the fuel. In atomization systems, on the other hand, the fuel droplets are broken down into fine liquid particles which are carried along by the airstream, though some amount of vaporization may also occur. However, the presence of liquid droplets or particles, no matter how fine, reduces the efficiency of combustion. Even when stoichiometric air-fuel ratios (i.e., wherein the air factor, λ (the ratio of quantity of air used/theoretical air requirement) is unity) are used, there is still an inefficiency in the combustion process. Generally this inefficiency can be explained in terms of the outer layers of each fuel droplet being progressively combusted, and thus the overall combustion process continues as the piston begins to travel in the power stroke. Accordingly, some of the combustion occurs at the lower pressures present during this downstroke, and thus the efficiency of the combustion process is diminished. To obtain optimum performance, all the combustion occurs at about top dead centre (TDC), where pressure is at its highest.
U.S. Pat. No. 4,372,278 describes a fuel system for an engine, in which fuel is said to be vaporized by a combination of high pressure and high temperature of the fuel itself, in contrast to many other types of ostensibly “vaporization systems”, which according to this reference are really atomization systems. Such high temperatures and pressures may reach 250° C. and 600 psi, respectively. However, the system described by U.S. Pat. No. 4,372,278 has a number of disadvantages. For example, heating the fuel to elevated temperatures and pressures requires the fuel system to be designed to withstand high temperature fuel circulating around the fuel system. Such a system increases capital costs of the engine, and moreover the high temperature and pressure of the fuel increases risk of damage and injury in case of accidents.
In U.S. Pat. No. 3,762,378 a fuel injection system is described in which fuel is injected directly into the combustion chamber of a cylinder. The fuel is preheated in the injector arrangement prior to injection into the cylinder. The fuel line to the injector includes a heat exchanger arrangement in the exhaust conduit of the cylinder, which also serves to preheat the fuel. The fuel is preheated to a temperature above its ignition temperature, and ignites in the combustion chamber as a result of then being introduced into an environment having a lower pressure than in the injector and oxygen to support combustion. A retractor valve in the fuel conduit to the injector maintains the fuel pressure in the injector above the critical pressure of the fuel but below the opening pressure of the valve head of the injector between pressure pulses.
U.S. Pat. No. 2,881,828 describes a fuel injection system for a diesel engine, in which the fuel temperature is controlled thermostatically by means of heat exchange between the inlet and outlet fuel lines to the injector.
In U.S. Pat. No. 3,738,334, a preheater arrangement heats fuel prior to the carburetor using a heat exchanger arrangement coupled to the exhaust gases.
In addition, prior art fuel heating systems also suffer from the following problem. By heating the fuel droplets to a high temperature, there increases the possibility of premature combustion of the droplets, even before they are fully atomized or vaporized. Pre-ignition problems may develop, and, the fuel is not fully burnt, leading to low running efficiency and an increase in the pollutants emitted to the atmosphere. Such problems are neither discussed nor hinted with respect to such prior art high temperature and pressure fuel heating systems.
In WO 00/25015, a device is provided for improving the atomization and mixing of fuel droplets in the air-fuel mixture upstream of the combustion chamber. Further, a combustion stability means is provided for providing an atomized medium to the combustion chamber during the induction stroke. The medium is described as comprising methanol and acetic acid, which is aerated and siphoned off to the air inlet system of the engine. The methanol improves the vaporization characteristics of the acetic acid. According to this aspect of the invention, an atomizer is provided for ensuring a high degree of atomization of the medium is provided. Such a system may generally help in cleaning parts of the engine and in reducing the possibility of pre-ignition of the fuel-air mixture.
Other devices for enhancing engine performance by providing water in a fine mist state are known, for example as disclosed by U.S. Pat. Nos. 3,767,172 and 4,076,002. However, while improving engine performance, use of water injection in internal combustion engines has certain drawbacks including the formation of calcium and slag deposits on the valves, pistons and spark plugs.